1. Field of the Invention
This invention relates to a single-phase brushless motor which is well adapted for a spindle motor for a floppy disk drive, a motor for audio and video appliances, and a motor for an axial-flow fan.
2. Description of the Prior Act
A brushless motor is used in a variety of appliances owing to its characteristics that it makes little noises and has a long life because it has neither brush nor commutator in addition to its characteristics as a dc motor. In a brushless motor, an electronic circuit is used for switching energization of armature coils of the motor and includes, for applicable phases of the motor, a corresponding number of driving circuits each including a position detecting element. Accordingly, it is a drawback that as the number of phases increases, the production cost increases accordingly due to such position detecting elements.
Therefore, inexpensive appliances such as axial-flow fans commonly employ a single-phase brushless motor which includes a driving circuit designed for a single-phase and hence can be produced at a low cost.
Such a single-phase brushless motor has a drawback that it cannot start itself if it stops at a dead point.
Therefore, a conventional single-phase brushless motor is normally provided with special means such as a cogging torque generating member such as an iron bar located in an air gap or a special stator armature providing a slanted or sloped air gap for generating a cogging torque to assure self-starting of a rotor of the motor.
Such conventional single-phase brushless motors utilizing a cogging torque as described above have drawbacks that smooth rotation cannot be attained due to generation of a cogging torque and that they are not easy to assemble and hence are expensive. Besides, the brushless motor employing an iron bar has a relatively large number of parts while the brushless motor employing a special stator armature cannot provide a high turning torque.